Sorry to be blunt, but sometimes that's just required.
You love two men. You're engaged to one of them, and telling him about loving two men may end your relationship with him.
Marrying Bob while hiding your relationship with John is cheating. It doesn't even matter if you never have sex with John, because it's emotional cheating. I'm not Christian and I don't know scripture, but I'm quite certain it has something to say about women who cheat on their husbands.
As for John being sent by the devil or a test from God, you're in the wrong forum to get confirmation on either of those theories. The belief around here is that it's absolutely awesome to love two people and there's nothing wrong with it!
I agree with the post that you should not enter a marriage if you have any doubts. Look at how torn you are right now, thinking you can't call off the wedding. How torn are you going to be in 6 months, a year, 10 years, when you still feel the same way about John, and you're MARRIED? As hard as it is to call off the wedding, won't it be a thousand times harder to get divorced?
And don't kid yourself. You won't survive the next 60 years if you force yourself to live in this lie.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker